Conventionally for many years buses are fabricated from a steel welded frame which is clad externally by sheets of aluminum or similar non-elastic cladding material bolted or riveted side by side over the steel frame. The interior surface is similarly clad by sheets of suitable material again attached to the steel frame. Insulation material is added as pieces inserted simply between the components of the frame and the interior and exterior cladding.
While a vehicle of this structure is relatively strong and resistant to impact damage, should a significant impact occur, there is a danger of penetration from the exterior through the interior cladding leaving a hole which can expose the passengers to danger. Any such perforated structure, even though it can in many cases still be driven cannot legally carry passengers and therefore must immediately be taken out of service.
In addition the repairs to the structure cannot be effected in the field and are relatively expensive since the individual panels must be replaced in a relatively time consuming operation at a service facility.